The parents of a two-year-old boy who died after drinking a beaker of the heroin substitute methadone have been convicted of manslaughter by a jury at Nottingham crown court.

Sally Dent, 33, and Shaun Binfield, 45 wept when the verdict was read out following a two-week trial examining the premature death of their son, Riley.

In what Judge John Milmo QC described as an emotionally charged and distressing case, the jury heard that Dent had a history of drug use going back many years and was prescribed methadone to try to rid her of her habit.

A green and yellow child's beaker of the substitute narcotic was left in the couple's bedroom on 12 March last year so Dent could drink from it during the night if she needed it. The next morning Dent, who had taken heroin the night before, woke to find Riley unconscious next to her in bed after the toddler drank 10ml to 20ml of the drug from the beaker.

The couple told the court a cup was usually kept on top of the wardrobe in their bedroom but had been moved on that occasion because it was more convenient for Dent.

Binfield told the court he forgot to move the cup because he had got caught up with putting the family bins out and the childcare routine after getting up.

He fought back tears as he admitted he missed three opportunities to move the cup.

Grabbing on to the sides of the witness box and struggling to keep his voice level as he answered questions under cross examination from prosecutor Yvonne Coen QC, he said: "If the alarm bells had rung I would have removed it and my son would still be here."

Coen asked him: "It's not the possibility then that you thought to yourself 'Ah, the methadone's in there still but it's safe', or are you saying 'I never even thought about the methadone at all'?"

Binfield replied: "I never even thought about it."

As Coen repeated further allegations that he forgot about the cup, Dent burst into tears and shouted from the courtroom dock: "How much pain do you want us to go through?"

Dent called 999 and paramedics arrived at the house to find Riley lifeless. His heart had stopped beating and his lips were blue, the court heard.

The child was taken to hospital but was pronounced dead shortly after arriving. Postmortem tests showed that Riley had enough methadone in his blood, stomach and urine to prove fatal.

Dent regularly sold some of her methadone to other drug addicts to make extra money and was struggling to kick her addiction, the court was told. Following her conviction, the court heard she had previous convictions for possessing Class A drugs with intent to supply, for assault and for prostitution.

The jury took just four hours to convict Dent and Binfield of manslaughter.

Dent was also found guilty of a separate charge of cruelty to a child under the age of 16 for failing to keep methadone out of reach. Judge Milmo adjourned sentencing until 19 February at Nottingham crown court.

Following the conviction, Detective Chief Inspector Phil Cox, of the East Midlands major crime unit, who was the investigating officer in the case, said: "Our sympathy goes out to everyone in Riley's family. The tragic death of this little boy was entirely avoidable. Methadone is a highly dangerous drug which never should have been put in a child's beaker. Drugs such as this should always be locked away and kept well out of the reach of children."